1. Field of the Invention
The present invention relates to an image forming apparatus that can use an electrophotographic process. More specifically though not exclusively, the present invention relates to a color image forming apparatus that can form color images of different colors by using a plurality of laser beams.
2. Description of the Related Art
The relationship between an electric current that is supplied to a laser diode and its optical output differs in accordance with the type of laser diode. Further, the relationship between the current and the optical output differs also in accordance with a heat generated by the laser diode itself. Thus, a laser cannot be appropriately emitted by an open loop constant current control. Therefore, it can be necessary in some circumstances to perform a control so that a desired level of the optical output can be obtained by monitoring the output of the laser. The control is referred to as an automatic power control (APC). FIG. 5 illustrates a laser control circuit that performs the APC.
The laser control circuit includes a constant current circuit 501, a switching circuit 502, and an amplifier 503. The constant current circuit 501 is a voltage-current converter that causes an electronic current I1 to flow in accordance with a light amount signal 504 from a control device (CPU) 510. The switching circuit 502 switches the current I1 by a laser lighting signal 505. A laser diode 506 emits a light in accordance with an operation of the switching circuit 502.
An amount of the emission of light from the laser diode 506 is detected as an electronic current value by a light detector 507, and the detected current value is converted into a voltage value by a resistor 508. The light emission amount that has been converted into the voltage value is amplified by the amplifier 503, and converted into a light emission amount signal 509. The control device 510 monitors the light emission amount signal 509 and increases/decreases the level of the light amount signal 504.
FIG. 6 is a flow chart that illustrates the APC operation. After the start of the APC (S1) and after causing a laser compulsory lighting signal to be active, the light emission amount signal is monitored (S2). If the light emission amount signal (also referred to as the light amount signal) is lower than a desired value, the level of the light amount signal is increased by one step (S3). On the other hand, if the light emission amount signal is higher than the desired value, the level of the light amount signal is decreased by one step (S5). Then, when the level of the light emission amount signal is adjusted to the desired value, the APC operation ends (S4).
FIG. 7 illustrates a timing of the APC operation using a semiconductor light emitting element that emits a plurality of laser beams. “APC 1” denotes the APC operation timing of a beam 1, and “APC 2” denotes the APC operation timing of a beam 2.
As illustrated in FIG. 7, APC2 is performed at a first predetermined timing, and next, APC1 is performed at a second predetermined timing. Contemporaneously, a main scanning synchronization signal BD1 of the beam 1 is detected. Then, when a main scanning synchronization signal BD2 of the beam 2 is detected, the APC 2 or a laser light emission ACC2 (hereinafter simply referred to as ACC) in which no APC is performed is carried out. Thus, the APC is performed to each laser for each line.
Further, as illustrated in FIG. 8, the APC can be performed between paper sheets.
Further, as illustrated in FIG. 9, there is a case where APC operations are repeatedly and alternatively performed for each line in an order of APC1, APC2, APC1. When the main scanning synchronization signal is detected, only the beam 1 is detected by a laser light emission ACC1 which performs no APC1.
Japanese Patent Application Laid-Open No. 11-101947 discusses related art thereto.
However, in the conventional technologies, the APC is performed for each laser beam per each line. Therefore, the time required for the APC can be lengthy for some uses, because it takes twice as a long time as a one-beam laser in the case of a two-beam laser, and four times as long as a one beam laser in the case of a four-beamlaser. Therefore, when the time spent for the APC is limited, it can be difficult to effectively and appropriately perform the APC.
In addition, in the case where the APC is performed between the paper sheets, the light amount for the overall area of the sheet may not be appropriately secured. Therefore, the density of an image is not the same on the paper between where printing started and where printing ends.
In addition, there is a case where the APC operations are repeatedly and alternatively carried out for each line in an order such as APC1, APC2, APC1, . . . and only the beam 1 is detected by the laser mission control. In this laser emission, no APC is performed when the main scanning synchronization signal is detected. In such a case, since the time for detecting the main scanning synchronization has to be taken, the APC cannot be performed in a sufficiently long time period.